The invention relates to a method for preventing circular routing in a telecommunication network.
In principle, the solution of the invention is applicable to any communication network, but is, however, primarily intended to improve the management of routing in a common channel signalling system. This signalling system is described in detail in the CCITT blue book Specifications of Signalling System No. 7 (Melbourne 1988).
In common channel signalling, the method of the invention is limited particularly to the message transfer part (MTP) acting as the transmission system of this network. MTP transfers signalling messages between locations of user functions.
Circular routing refers to a situation where messages sent in a network start to circle in a loop and never reach the destination node. The messages may then even return to the originating node for onward routing towards the same destination.
Point 13 in the CCITT recommendation Q.704 proposes that particular management messages be used for notifying of the unavailability, restriction and availability of a signalling route. These messages include e.g:
transfer-prohibited (TFP) messages whereby the node acting as a signalling transfer point notifies adjacent signalling points that they must no longer route signalling messages via that transfer point, PA1 transfer-allowed (TFA) messages whereby the node acting as a signalling transfer point notifies adjacent signalling points that they may route signalling messages via that transfer point and, PA1 transfer-restricted (TFR) messages whereby the node acting as a signalling transfer point notifies adjacent signalling points that they should, if possible, no longer route signalling messages via that transfer point. According to the recommendation, transfer-restricted messages are a national option.
Below, transfer-prohibited messages are referred to as TFP messages, transfer-allowed messages as TFA messages and transfer-restricted messages as TFR messages. As a signalling network is concerned, the nodes are also referred to as signalling points. For a more detailed description of the messages, reference is made to the above-identified CCITT recommendation.
The problem of the procedures described in the CCITT recommendation is that the TFP messages prevent a loop between two points but are unable to prevent loops involving three or more signalling points. This is illustrated by an exemplary network of FIGS. 1a through 1d comprising four signalling points A through D. A primary route, denoted by reference I, and a secondary (standby) route, denoted by reference II, originates from each signalling point. (This example, where standby routes form a loop, cf FIG. 1a, is quite classical.) Following notations are used in the description:
______________________________________ A: B AV-EX a route originating from point A passing subsequently via a channel system leading to point B is in status AV-EX. ______________________________________
The statuses of the routes are as follows:
______________________________________ AV-EX a route transmitting traffic, AV-SP an available standby route, UA-link an unavailable route because of channel system failure. ______________________________________
With all channel systems leading to point D available (FIG. 1a), routing information of the signalling network to point D is as follows:
______________________________________ (i) A: D AV-EX B: D AV-EX C: D AV-EX B AV-SP C AV-SP A AV-SP. ______________________________________
Should channel system C.fwdarw.D fail (FIG. 1b), signalling point C switches to channel system A and sends a TFP message (A.fwdarw.C.fwdarw.D) (a TFP message prohibiting message transmission from point A to point D via point C). The respective route is unavailable to signalling point A, and the routing status remains as:
______________________________________ (ii) A: D AV-EX B: D AV-EX C: D UA-link B AV-SP C AV-SP A AV-EX. ______________________________________
Should channel system B.fwdarw.D fail subsequently (FIG. 1c), signalling point B switches to channel system C and sends a TFP message (C.fwdarw.B.fwdarw.D). Signalling point C does not process the TFP message because the respective route is unavailable to it, and the current routing status is:
______________________________________ (iii) A: D AV-EX B: D UA-link C: D UA-link B AV-SP C AV-SP A AV-EX. ______________________________________
Should channel system A.fwdarw.D fail subsequently (FIG 1d), the present CCITT standards lead to circular routing. Now signalling point A switches to channel system B and sends a TFP message (B.fwdarw.A.fwdarw.D). Signalling point B does not process the TFP message because the respective route is unavailable to it, and the final routing is:
______________________________________ (iv) A: D UA-link B: D UA-link C: D UA-link B AV-EX C AV-EX A AV-EX. ______________________________________
The above describe scenario results in the messages addressed to signalling point D circulating in a loop between points A, B and C until overload causes intra-nodal restarts. These are of no help, and routing always returns to the status (iv).
The above described problem has been recognized and some methods have been realized for preventing the above described circular routing. An improved method (presented by British Telecom) is based on the signalling point sending more frequent TFP messages; having started to direct signalling traffic destined to point X to a standby route, the signalling point (node) sends a TFP message concerning destination point X to all adjacent nodes. When switching onto a standby route, the node in fact prohibits its adjacent nodes from using itself as a transfer point. This prevents circular routing unless a loop is formed from primary routes, but often results in the signalling points being unnecessarily inaccessible.
The problems of the latter method are disclosed in a very simple and realistic network presented in FIGS. 2a and 2b, where no circular routing could even occur. Signalling point A (i.e. node A), located furthermost from signalling point D, routes signalling messages destined to point D via point B. Point B may convey the messages to point D either directly or, in an emergency, via point C.
With all channel systems leading to point D available (FIG. 2a), the routing information of the signalling network to point D is (using the above notations):
______________________________________ (i) A: B AV-EX B: D AV-EX C: D UA-Ex. C AV-SP ______________________________________
Should channel system B.fwdarw.D fail (FIG. 2b), signalling point B switches to channel system C and notifies all adjacent points of switching onto a standby route by sending messages TFP(A.fwdarw.B.fwdarw.D) and TFP(C.fwdarw.B.fwdarw.D). Having received the message TFP(A.fwdarw.B.fwdarw.D), signalling point A recognizes that point D is no longer accessible and sends, in turn, a message TFP(B.fwdarw.A.fwdarw.D) (the latter transmission is based on the transfer-prohibited transmission method of the CCITT specification Q.704 point 13.2.2 ii) the final routing status is thus:
______________________________________ (ii) A: D UA-TFP B: D DA-link C: D AV-EX. C AV-EX ______________________________________
where UA-TFP indicates that the route is unavailable because of a TFP message.
In the above described method, wherein a TFP message is always sent when switching onto a standby route, a signalling point (point D above) is unnecessarily made inaccessible.